Process for producing semiconductor member, process for producing solar cell, and anodizing apparatus

ABSTRACT

In a process for producing a semiconductor member, and a solar cell, making use of a thin-film crystal semiconductor layer, the process comprises the steps of: (1) anodizing the surface of a first substrate to form a porous layer at least on one side of the substrate, (2) forming a semiconductor layer at least on the-surface of the porous layer, (3) removing the semiconductor layer at its peripheral region, (4) bonding a second substrate to the surface of the semiconductor layer, (5) separating the semiconductor layer from the first substrate at the part of the porous layer, and (6) treating the surface of the first substrate after separation and repeating the above steps (1) to (5).  
     This process enables separation of the thin-film semiconductor layer at a small force while causing less cracks, breaks or defects to be brought into it and can manufacture high-performance semiconductor members and solar cells in a good efficiency.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of separating a semiconductorthin film deposited on a porous layer, a process for producing asemiconductor member, a process for producing a solar cell formed of athin-film single crystal layered on a low-cost substrate, and ananodizing apparatus used in these.

[0003] 2. Related Background Art

[0004] A technique is known in which a thin-film semiconductor layer isformed on a porous layer formed at the surface portion or layer of asemiconductor substrate and thereafter the semiconductor layer isseparated at the part of the porous layer. As a separation method, usedis a chemical method making use of etching or a physical method ofcausing ultrasonic waves or force such as tensile force to act on.

[0005] With regard to the physical method, Japanese Patent ApplicationLaid-Open No. 7-302889 discloses that a porous layer is formed at thesurface of a silicon wafer, thereafter an epitaxial growth layer isformed thereon, another wafer is bonded to the epitaxial growth layer(silicon layer), and a pressure, a shear force or ultrasonic wavesis/are applied to the porous layer to make separation. Japanese PatentApplication Laid-Open No. 8-213645 also discloses that a porous layer isformed at the surface of a single-crystal silicon substrate, thereaftera p-n junction layer is formed thereon, the single-crystal siliconsubstrate is, on its back, fastened to a jig through an adhesive,another jig is bonded to the epitaxial growth layer, and both the jigsare pulled against each other to cause the porous layer to break toobtain a thin-film epitaxial growth layer (a solar cell). JapanesePatent Application Laid-Open No. 10-190032 discloses that a differencein shrink between a silicon layer and a plastic substrate bonded to thesilicon layer is utilized to separate the former from the latter bycooling them with a vapor of liquid nitrogen.

[0006] However, when the thin-film epitaxial growth layer is obtained byseparating it at the part of the porous layer, the thin-filmsemiconductor layer may finely be cracked or broken on the periphery ofa region where it is to be separated (i.e., a separating region),because of an impact produced when the porous layer formed at thesurface of the first substrate is broken by separating force. Where itis thus cracked or broken, not only the thin film can be handled withdifficulty but also, when cracked or broken up to the central area, theyield and characteristics of devices including solar cells may lower.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a process forproducing a semiconductor member and a solar cell, which process enablesseparation of the thin-film semiconductor layer at a small force whilecausing less cracks, breaks or defects to be brought into it and canmanufacture high-performance semiconductor members and solar cells in agood efficiency.

[0008] To achieve the above object, the present invention provides aprocess for producing a semiconductor member making use of a thin-filmcrystal semiconductor layer, the process comprising the steps of:

[0009] (1) anodizing the surface of a first substrate to form a porouslayer at least on one side of the substrate;

[0010] (2) forming a semiconductor layer at least on the surface of theporous layer;

[0011] (3) removing the semiconductor layer at its peripheral region;

[0012] (4) bonding a second substrate to the surface of thesemiconductor layer;

[0013] (5) separating the semiconductor layer from the first substrateat the part of the porous layer by applying an external force to atleast one of the first substrate, the porous layer and the secondsubstrate; and

[0014] (6) treating the surface of the first substrate after separationand repeating the above steps (1) to (5).

[0015] The present invention also provides a process for producing asemiconductor member making use of a thin-film crystal semiconductorlayer, the process comprising the steps of:

[0016] (1) anodizing the surface of a first substrate to form a porouslayer at least on one side of the substrate;

[0017] (2) forming a semiconductor layer at least on the surface of theporous layer;

[0018] (3) bonding a second substrate to the semiconductor layer;

[0019] (4) removing the semiconductor layer at its region not coveredwith the second substrate;

[0020] (5) separating the semiconductor layer from the first substrateat the part of the porous layer by applying an external force to atleast one of the first substrate, the porous layer and the secondsubstrate; and

[0021] (6) treating the surface of the first substrate after separationand repeating the above steps (1) to (5).

[0022] The present invention still also provides a process for producinga solar cell making use of a thin-film crystal semiconductor layer, theprocess comprising the steps of:

[0023] (1) anodizing the surface of a first substrate to form a porouslayer at least on one side of the substrate;

[0024] (2) forming a semiconductor layer at least on the surface of theporous layer;

[0025] (3) removing the semiconductor layer at its peripheral region;

[0026] (4) bonding a second substrate to the surface of thesemiconductor layer;

[0027] (5) separating the semiconductor layer from the first substrateat the part of the porous layer by applying an external force to atleast one of the first substrate, the porous layer and the secondsubstrate; and

[0028] (6) treating the surface of the first substrate after separationand repeating the above steps (1) to (5).

[0029] The present invention further provides a process for producing asolar cell making use of a thin-film crystal semiconductor layer, theprocess comprising the steps of:

[0030] (1) anodizing the surface of a first substrate to form a porouslayer at least on one side of the substrate;

[0031] (2) forming a semiconductor layer at least on the surface of theporous layer;

[0032] (3) bonding a second substrate to the semiconductor layer;

[0033] (4) removing the semiconductor layer at its region not coveredwith the second substrate;

[0034] (5) separating the semiconductor layer from the first substrateat the part of the porous layer by applying an external force to atleast one of the first substrate, the porous layer and the secondsubstrate; and

[0035] (6) treating the surface of the first substrate after separationand repeating the above steps (1) to (5).

[0036] The present invention still further provides a process forproducing a semiconductor member obtained by separating a thin-filmcrystal semiconductor layer formed on a first substrate to transfer theformer to a second substrate, wherein the thin-film crystalsemiconductor layer is removed by etching by electropolishing at itspart on the periphery of the first substrate.

[0037] The present invention still further provides a process forproducing a semiconductor member making use of a thin-film crystalsemiconductor layer, the process comprising the steps of:

[0038] (1) anodizing the surface of a first substrate at least on itsprincipal-surface side to form a porous layer;

[0039] (2) forming a semiconductor layer on the surface of the porouslayer;

[0040] (3) removing the semiconductor layer at its part on the peripheryof the first substrate by electropolishing;

[0041] (4) bonding a second substrate to the surface of thesemiconductor layer;

[0042] (5) separating the semiconductor layer from the first substrateat the part of the porous layer to transfer the semiconductor layer tothe second substrate; and

[0043] (6) treating the surface of the first substrate after separationand repeating the above steps (1) to (5).

[0044] The present invention still further provides a process forproducing a solar cell obtained by separating a thin-film crystalsemiconductor layer formed on a first substrate to transfer the formerto a second substrate, wherein the thin-film crystal semiconductor layeris removed by etching by electropolishing at its part on the peripheryof the first substrate.

[0045] The present invention still further provides a process forproducing a solar cell making use of a thin-film crystal semiconductorlayer, the process comprising the steps of:

[0046] (1) anodizing the surface of a first substrate at least on itsprincipal-surface side to form a porous layer;

[0047] (2) forming a semiconductor layer on the surface of the porouslayer;

[0048] (3) removing the semiconductor layer and the porous layer attheir part on the periphery of the first substrate by electropolishing;

[0049] (4) forming a surface anti-reflection layer on the surface of thesemiconductor layer at its part other than that on the periphery of thefirst substrate;

[0050] (5) bonding a second substrate to the surface of thesemiconductor layer;

[0051] (6) separating the semiconductor layer from the first substrateat the part of the porous layer to transfer the semiconductor layer tothe second substrate; and

[0052] (7) treating the surface of the first substrate after separationand repeating the above steps (1) to (6).

[0053] The present invention still further provides an anodizingapparatus comprising, at the peripheral portion of a substrate to besubjected to anodizing, a first electrode coming in contact with theback side of the substrate and a second electrode facing the firstelectrode, interposing the substrate between them; the first electrodehaving substantially the same form as the second electrode.

[0054] The present invention still further provides an anodizingapparatus comprising, at the peripheral portion of a substrate to besubjected to anodizing, a first electrode coming in contact with theback side of the substrate and a second electrode facing the firstelectrode, interposing the substrate between them, and, in the remainingsubstrate region excluding the peripheral portion, a third electrodecoming in contact with the back side of the substrate and a fourthelectrode facing the third electrode, interposing the substrate betweenthem; the first electrode and third electrode having substantially thesame form as the second electrode and fourth electrode, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G and 1H illustrate an example of athin-film semiconductor production process according to the presentinvention, which is a process carried out in Example 1.

[0056]FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G and 2H illustrate another exampleof a thin-film semiconductor production process according to the presentinvention, which is a process carried out in Example 2.

[0057]FIGS. 3A and 3B illustrate how an epitaxial layer is cracked orbroken when separated.

[0058]FIG. 4 is a cross-sectional view of a separating jig used inExample 3.

[0059]FIG. 5 is a schematic view of a holder portion of a peripheryetching apparatus used in Example 3.

[0060]FIG. 6 is a cross-sectional view of a back-sidejunction-concentrated solar cell formed in Example 4.

[0061]FIG. 7 is a plan view showing a periphery-removing portion of thesolar cell formed in Example 4.

[0062]FIG. 8 is a plan view showing an isolation region and aperiphery-removing portion of a solar cell formed in Example 5.

[0063]FIG. 9 illustrates a method of removing a porous layer formed atan edge of a wafer, carried out in Example 6.

[0064]FIGS. 10A, 10B, 10C, 10D, 10E, 10F, 10G and 10H illustrate asemiconductor member production process according to the presentinvention.

[0065]FIGS. 11A, 11B, 11C, 11D, 11E, 11F, 11G, 11H and 11I illustrate asolar cell production process according to the present invention.

[0066]FIGS. 12A, 12B, 12C, 12D, 12E, 12F, 12G, 12H and 12I illustrateanother solar cell production process according to the presentinvention.

[0067]FIG. 13 illustrates the construction of an anodizing apparatus ofthe present invention.

[0068]FIG. 14 illustrates the construction of another anodizingapparatus of the present invention.

[0069]FIG. 15 illustrates the construction of still another anodizingapparatus of the present invention.

[0070]FIG. 16 illustrates examples of a form an electrode for removingthe peripheral portion may have, used in the anodizing apparatus of thepresent invention.

[0071]FIG. 17 illustrates other examples of a form an electrode forremoving the peripheral portion may have, used in the anodizingapparatus of the present invention.

[0072]FIG. 18 illustrates another example of a form an electrode forremoving the peripheral portion may have, used in the anodizingapparatus of the present invention.

[0073]FIG. 19 illustrates still another example of a form an electrodefor forming an anti-reflection layer may have, used in the anodizingapparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] Embodiments of the present invention will be described below indetail.

[0075] (Embodiment 1)

[0076] As an embodiment according to the present invention, a processfor producing a semiconductor member is described with reference toFIGS. 1A to 1H.

[0077] Into the surface portion or layer of a first-substrate crystalsubstrate as exemplified by a single-crystal silicon wafer 101,impurities are introduced by thermal diffusion or ion implantation orare incorporated when the wafer is produced, to form a p⁺-type (orn⁺-type) layer 102 at least at the wafer surface (FIG. 1A).

[0078] Next, the wafer surface on the side the impurities have beenintroduced is subjected to anodizing in, e.g., an aqueous HF (hydrogenfluoride) solution to make the surface and the vicinity thereof porousto form a porous layer 103 (FIG. 1B).

[0079] This porous layer 103 is subjected to hydrogen annealing to makeits surface smooth, followed by CVD (chemical vapor deposition) orliquid-phase epitaxial growth to grow a single-crystal siliconsemiconductor layer 104 (FIG. 1C).

[0080] In the course of the anodizing to make porous, the level ofanodizing electric current may be changed, e.g., from a low level to ahigh level. This makes it possible to previously provide the porouslayer with a structural change in density, whereby after epitaxialgrowth the semiconductor layer 104 can be separated (peeled) from thesilicon wafer 101 with ease at the part of the porous layer.

[0081] Where the side of a wafer on which the semiconductor layer isformed is defined to be the surface and the other side the back, thesemiconductor layer 104 formed on the porous layer 103 comes as follows:When, e.g., the surface portion is made porous in the state theperiphery of the surface is shielded from the anodizing solution at thetime of anodizing, and the epitaxial growth is carried out on the wholesurface, what is formed in and on the wafer are, as shown in FIG. 3A, aporous layer 303, a flat single-crystal layer 305 formed when pores ofthe porous-layer surface portion are stopped up as a result of thehydrogen annealing carried out before the semiconductor layer is formed,and a single-crystal silicon semiconductor layer 304 formed by epitaxialgrowth. In order to separate the semiconductor layer to transfer it to asecond substrate 308, the semiconductor layer 304, single-crystal layer305 and porous layer 303 must be broken by separating force so as toreach a portion having the lowest breaking strength in the porous layer,and at this time the semiconductor layer 304 tends to be cracked orbroken or to have other defects brought into it.

[0082] As another example, when the porous layer is formed in the statethe back is shielded from the anodizing solution, and the epitaxialgrowth is carried out in the state the wafer (with the porous layer) ismasked on the periphery of the surface up to the whole back, a structureas shown in FIG. 3B comes about. The porous layer 303 is nothydrogen-annealed at its masked portion and hence neither flatsingle-crystal layer 305 nor semiconductor layer 306 is formed at thatportion. In order to separate the semiconductor layer 304 to transfer itto the second substrate 308, the porous layer must be broken at its someportion so that the thin-film single-crystal silicon layer can beseparated at a portion having the lowest breaking strength in the porouslayer. In this case, too, when the porous layer has a high breakingstrength, the single-crystal silicon layer 304 formed by epitaxialgrowth may be damaged like the above case.

[0083] Accordingly, depending on the separation strength of the porouslayer, only the single-crystal silicon layer (silicon layer 104) or, inaddition thereto, part or the whole of the porous layer at its/theirportion(s) lying on the periphery of the separating region is/areremoved (FIG. 1D). Then, a second substrate supporting substrate 106 isbonded to the silicon layer 104 via an adhesive layer 105 such that itis not bonded to the portion uncovered after removal (FIG. 1E).Thereafter, a physical separating force (e.g., a direct force such asmechanical force or an indirect force that acts via a medium, such asultrasonic waves) is applied to the porous layer 103 to separate thesilicon layer from the silicon wafer 101 and transfer it onto thesupporting substrate 106 (FIG. 1F). In this case, the direct force maydirectly be applied to the porous layer 103, or may be applied to one orboth of the wafer 101 and the supporting substrate 106, or may beapplied to all of them.

[0084] By doing so, the thin-film semiconductor layer can be made lessbroken or damaged and also any force applied to the porous layer at itsportion other than the part where it is readily separable can be madesmall, making it possible to effect separation at a small force. Here,the silicon layer and optionally the porous layer may be removed afterthe supporting substrate 106 has been bonded. In this case, it isefficient to use the supporting substrate as a substitute for a masknecessary for removing the periphery (see FIGS. 2A to 2H). The removingof the semiconductor layer and optionally the porous layer at its/theirportion(s) lying on the periphery of the separating region, which may beeffective when done at part of the periphery, can be more effective whendone on the whole periphery.

[0085] A porous residue 107 remaining after transfer on the surface ofthe thin-film single-crystal silicon layer may optionally be etched awayby etching or the like to obtain a semiconductor member or solar cell(FIG. 1G). The supporting substrate used for the separation of thesilicon layer may be incorporated in the semiconductor member or solarcell product as it is, or the thin-film silicon may again be transferredto a third substrate suited for the product.

[0086] The silicon wafer 101 from which the silicon layer has beenseparated may be treated to remove by etching or the like the porousresidue 107 remaining on its surface. Thus, it can be reused in thefirst step and can effectively be utilized (FIG. 1H).

[0087] The removing of the periphery of the separating region, which isto obtain a thin-film semiconductor with ease, may be done before thesubstrate for supporting the silicon thin film (the supportingsubstrate) is bonded or before the supporting substrate has been bonded.In the case where the periphery of the separating region is removedbefore the supporting substrate is bonded, it is removed by dry etchingsuch as reactive ion etching, wet etching or electrolytic etching makinguse of a hydrofluoric acid type etchant, mechanical methods such asgrinding or polishing, or laser-assisted etching, in the state theseparating region is masked. Thereafter, the supporting substrate isbonded to the separating region while paying attention so as not toprovide the adhesive to the portion uncovered after removal. In the casewhere the periphery is removed after the supporting substrate has beenbonded, the supporting substrate for the thin film may be made to serveas a mask, thereby making good use of the material and saving the stepof removing the mask, resulting in a good efficiency.

[0088] What is removed is only the peripheral region of thesemiconductor layer or, in addition thereto, part or the whole of theperipheral region of the porous layer. Since the porous layer may have adifferent structure depending on anodizing conditions and may have adifferent separation strength, it may be removed in a depth most suitedfor the separation.

[0089] Features of the process for producing the semiconductor memberand solar cell according to the present invention will be describedbelow in detail.

[0090] The porous layer is described first, taking the case of siliconas an example. In the anodizing for forming the porous layer (poroussilicon layer) 103, the aqueous HF solution (hydrofluoric acid) maypreferably be used. An aqueous hydrogen chloride solution (hydrochloricacid) or a solution of sulfuric acid may also be used. In the case wherethe aqueous HF solution is used, the p⁺-type (or n⁺-type) layer 102 canbe made porous when it has an HF concentration of at least 10% byweight. The quantity of electric current flowed at the time of anodizingmay appropriately be determined in accordance with the HF concentration,the intended layer thickness of the porous silicon layer and the stateof porous layer surface. Stated roughly, it may suitably be within therange of from 1 mA/cm² to 100 mA/cm².

[0091] An alcohol such as ethyl alcohol may also be added to the aqueousHF solution, whereby bubbles of reaction product gases generated at thetime of anodizing can instantaneously be removed from the reactionliquid surface without stirring and the porous silicon can be formeduniformly and in a good efficiency. The quantity of the alcohol to beadded may appropriately be determined in accordance with the HFconcentration, the intended layer thickness of the porous silicon layerand the state of porous layer surface. It must be determined especiallywhile paying attention not to make the HF concentration too low.

[0092] The single-crystal silicon has a density of 2.33 g/cm³. Thedensity of single-crystal silicon can be changed within the range offrom 1.1 to 0.6 g/cm³ by changing the concentration of the aqueous HFsolution from 50 to 20% by weight. Also, its porosity can be changed bychanging anodizing electric currents, where the porosity increases withan increase in electric currents.

[0093] Mechanical strength of porous silicon differs depending on theporosity, and is considered to be sufficiently lower than that of bulksilicon. For example, porous silicon having a porosity of 50% may beestimated to have a mechanical strength of half that of the bulksilicon. Assume that a substrate is bonded to the surface of a poroussilicon layer formed at the surface of a silicon wafer and a sufficientbonding power has been given between the porous silicon layer and thesubstrate. In such a case, the porous silicon layer is broken uponapplication of an instantaneous separating force such as compression,tension or shear force to the interface between the silicon wafer andthe substrate. Also, a like effect is obtainable when an indirect forceis made to act between them by externally applying energy such as heat,ultrasonic waves or centrifugal force. Still also, the porous siliconlayer can be broken by a weaker force or energy when made to have ahigher porosity.

[0094] It is reported that, in the formation of porous silicon byanodizing, the anodizing reaction requires holes and hence p-typesilicon, in which holes are chiefly present, is used to make the poroussilicon (T. Unagami, J. Electrochem. Soc., Vol.127, 476, 1980). On theother hand, however, there is another report that silicon can also bemade porous as long as it is a low-resistance n-type silicon (R. P.Holmstrom and J. Y. Chi, Appl. Phys. Lett., Vol.42, 386, 1983). Thus,without regard to whether the silicon is p-type of n-type, it can bemade porous using low-resistance silicon. Also, it can be made porousselectively in accordance with its conductivity type, and only thep-type layer can be made porous by carrying out anodizing in the dark asin the FIPOS (full isolation by porous oxidized silicon) process.

[0095] In porous silicon obtained by anodizing single-crystal silicon,pores having a diameter of few nanometers are formed as seen inobservation with a transmission electron microscope, and the poroussilicon has a density half or less that of the single-crystal silicon.Nevertheless, it is kept to stand single-(or mono)crystalline, where anepitaxial layer can be made to grow on the porous silicon by, e.g.,thermal CVD.

[0096] The porous layer also has voids which are formed in a largequantity in its interior, and hence has come to have a dramaticallylarge surface area compared with its volume. As a result, the rate ofits chemical etching can greatly be higher than the etching rate onusual single-crystal layers.

[0097] The porous layer is also obtainable similarly by anodizing evenwhen polycrystalline silicon is used in place of single-crystal silicon.On that layer, a single-crystal silicon layer can be formed by, e.g.,thermal CVD. (In this case, partial epitaxial growth is possible whichcorresponds to the size of crystal grains of the polycrystallinesilicon.)

[0098] To form the thin-film semiconductor layer, liquid-phase epitaxyand gas-phase epitaxy may be used.

[0099] (Embodiment 2)

[0100] As another embodiment according to the present invention, asemiconductor member production process employing electropolishing isdescribed with reference to FIGS. 10A to 10H.

[0101] As shown in FIG. 10A, first, into the surface portion (surfacelayer) of a single-crystal silicon substrate 1101, B (boron) isintroduced by thermal diffusion or ion implantation or is incorporatedwhen the substrate (wafer) is produced. The single-crystal siliconsubstrate the surface layer (1202) of which has become p⁺-type issubjected to anodizing in, e.g., an aqueous HF solution to make thep⁺-type surface layer 1202 porous to form a porous layer 1103 (FIG.10B). Here, the layer may be made porous such that the anodizing iscarried out first at a level of low electric current and, after lapse ofa certain time, at a level abruptly raised to high electric current andfor a short time. This makes it possible to previously provide theporous layer with an internal structural change in density, whereby in alater step a silicon layer 1104 can be separated from the single-crystalsilicon substrate 1101 with ease.

[0102] Next, on the surface layer 1103 thus made porous, the siliconlayer 1104 is formed by, e.g., thermal CVD (FIG. 10C). Here, at the timethe silicon layer 1104 is formed, a dopant may be introduced in a tracequantity to control the silicon layer to be of a p⁻-type (or n⁻-type).

[0103] The single-crystal silicon substrate 1101 having the siliconlayer 1104 is set in an anodizing apparatus shown in FIG. 14, at itsprescribed position such that the silicon layer 1104 faces a negativeelectrode 1504 in the aqueous HF solution. Here, the negative electrode1504 has substantially the same form as a positive electrode 1505 comingin contact with the back of the single-crystal silicon substrate 1101,and is provided along the periphery of the single-crystal siliconsubstrate 1101 in the form of, e.g., a beltlike ring or polygon as shownin FIG. 16. An electric current is flowed across the electrodes andetching is carried out in an electropolishing mode to remove the siliconlayer 1104, or the silicon layer 1104 and part or the whole of theporous layer 1103, lying on the periphery of the single-crystal siliconsubstrate 1101. In this case, the removing of either only the siliconlayer 1104 or both the silicon layer 1104 and part or the whole of theporous layer 1103 may be selected in accordance with the separatingstrength of the porous layer.

[0104] In the case where as shown in FIG. 3B the silicon layer 1104(304) does not completely cover the porous layer 1103 (303) and theporous layer 1103 is uncovered to the surface of the substrate 1101(wafer), part or the whole of the porous layer 1103 at its uncoveredportion is removed (FIG. 10D).

[0105] After an oxide film 1106 is formed at the surface of the siliconlayer 1104, a supporting substrate 1105 is bonded to the oxide film1106, and these are put in a heat-treating furnace (not shown) to bringthe supporting substrate 1105 and the silicon layer 1104 on thesingle-crystal silicon substrate 1101 into firm bond (FIG. 10E).

[0106] Next, force is made to act between the supporting substrate 1105and the single-crystal silicon substrate 1101 in the direction wherethey are pulled apart from each other, to separate them at the part ofthe porous layer 1103 (FIG. 10F).

[0107] A porous layer 1103 a remaining on the silicon layer 1104separated from the single-crystal silicon substrate 1101 is selectivelyremoved. Only the porous silicon can be removed by electroless wetchemical etching by the use of at least one of a usual silicon etchant,porous-silicon selective-etchant hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to hydrofluoric acid, buffered hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to buffered hydrofluoric acid, and an alkali solution of KOH, NaOHor hydrazine (FIG. 10G). The supporting substrate to which the siliconlayer has been transferred may be used as a semiconductor substrate asit is, or, as occasion calls, the silicon layer may again be transferredto a third substrate suited for products.

[0108] The single-crystal silicon substrate 1101 after separation may betreated in the same manner as the above to remove a porous layer 1103 bremaining on its surface. In a case where the surface is too rough forits flatness to be tolerable, the surface may optionally be flatted(FIG. 10H), and thereafter the substrate is reused in the step of FIG.10A.

[0109] (Embodiment 3)

[0110] As still another embodiment according to the present invention, athin-film crystal solar cell production process employingelectropolishing is described with reference to FIGS. 11A to 11I.

[0111] As shown in FIG. 11A, first, into the surface layer of asingle-crystal silicon substrate 1201, B (boron) is introduced bythermal diffusion or ion implantation or is incorporated when thesubstrate (wafer) is produced. The single-crystal silicon substrate thesurface layer (1202) of which has become p⁺-type is subjected toanodizing in, e.g., an aqueous HF solution to make the p⁺-type surfacelayer 1202 porous to form a porous layer 1203 (FIG. 11B). Here, thelayer may be made porous such that the anodizing is carried out first ata level of low electric current and, after lapse of a certain time, at alevel abruptly raised to high electric current and for a short time.This makes it possible to previously provide the porous layer with aninternal structural change in density, whereby in a later step a siliconlayer 1204 can be separated from the single-crystal silicon substrate1201 with ease.

[0112] Next, on the surface layer 1203 thus made porous, the siliconlayer 1204 is formed by, e.g., thermal CVD (FIG. 11C). Here, at the timethe silicon layer 1204 is formed, a dopant may be introduced in a tracequantity to control the silicon layer to be of a p⁻-type (or n⁻-type).On the silicon layer 1204, a p⁺-type layer (or n⁺-type layer) 1205 isdeposited by plasma CVD or by increasing the dopant when the formationof the silicon layer 1204 is finished (FIG. 11D).

[0113] The single-crystal silicon substrate 1201 having the siliconlayers 1204 and 1205 is set in an anodizing apparatus shown in FIG. 14,at its prescribed position such that the silicon layer 1205 faces anegative electrode 1504 in the aqueous HF solution. Here, the negativeelectrode 1504 has substantially the same form as a positive electrode1505 coming in contact with the back of the single-crystal siliconsubstrate 1201, and is provided along the periphery of thesingle-crystal silicon substrate 1201 in the form of, e.g., a beltlikering or polygon as shown in FIG. 16. An electric current is flowedacross the electrodes and etching is carried out in an electropolishingmode to remove the silicon layers 1204 and 1205, or the silicon layers1204 and 1205 and part or the whole of the porous layer 1203, lying onthe periphery of the single-crystal silicon substrate 1201. In thiscase, the removing of either only the silicon layers 1204 and 1205 orboth the silicon layers 1204 and 1205 and part or the whole of theporous layer 1203 may be selected in accordance with the separatingstrength of the porous layer.

[0114] In the case where as shown in FIG. 3B the silicon layers 1204 and1205 (304) do not completely cover the porous layer 1203 (303) and theporous layer 1203 is uncovered to the surface of the single-crystalsilicon substrate 1201 (wafer), part or the whole of the porous layer1203 at its uncovered portion is removed (FIG. 11E).

[0115] A polymeric-film substrate 1206 on which a silver paste has beenprinted as a back electrode 1207 is bonded in close contact with theside of the single-crystal silicon substrate 1201 on which the siliconlayers 1204 and 1205 have been formed, and these are put in an oven (notshown) and heated to bring the polymeric-film substrate 1206 and thesilicon layer 1205 on the single-crystal silicon substrate 1201 intofirm bond (FIG. 11F).

[0116] Next, force is made to act between the polymeric-film substrate1206 and the single-crystal silicon substrate 1201 in the directionwhere they are pulled apart from each other. That is, the flexibility ofthe polymeric film is utilized and the both are slowly drawn off from anedge of the single-crystal silicon substrate 1201 to separate them atthe part of the porous layer 1203 (FIG. 11G).

[0117] A porous layer 1203 a remaining on the silicon layer 1204separated from the single-crystal silicon substrate 1201 is selectivelyremoved. Only the porous silicon is removed by electroless wet chemicaletching by the use of at least one of a usual silicon etchant,porous-silicon selective-etchant hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to hydrofluoric acid, buffered hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to buffered hydrofluoric acid, and an alkali solution of KOH, NaOHor hydrazine.

[0118] On the surface of the silicon layer 1204 from which the porouslayer has been removed, an n⁺-type (or p⁺-type) layer 1208 is formed andfurther thereon a transparent conductive film (ITO) serving also as asurface anti-reflection layer and a grid type collector electrode 1210are formed by vacuum deposition to make up a solar cell (FIG. 11H).

[0119] The single-crystal silicon substrate 1201 after separation may betreated in the same manner as the above to remove a porous layer 1203 bremaining on its surface. In a case where the surface is too rough forits flatness to be tolerable, the surface may optionally be flatted(FIG. 11I), and thereafter the substrate is reused in the step of FIG.11A.

[0120] (Embodiment 4)

[0121] As a further embodiment according to the present invention, athin-film crystal solar cell production process employingelectropolishing is described with reference to FIGS. 12A to 12I.

[0122] As shown in FIG. 12A, first, into the surface layer of asingle-crystal silicon substrate 1301, B (boron) is introduced bythermal diffusion or ion implantation or is incorporated when thesubstrate (wafer) is produced. The single-crystal silicon substrate thesurface layer (1302) of which has become p⁺-type is subjected toanodizing in, e.g., an aqueous HF solution to make the p⁺-type surfacelayer 1302 porous to form a porous layer 1303 (FIG. 12B). Here, thelayer may be made porous such that the anodizing is carried out first ata level of low electric current and, after lapse of a certain time, at alevel abruptly raised to high electric current and for a short time.This makes it possible to previously provide the porous layer with aninternal structural change in density, whereby in a later step a siliconlayer 1304 can be separated from the single-crystal silicon substrate1301 with ease.

[0123] Next, on the surface layer 1303 thus made porous, the siliconlayer 1304 is formed by, e.g., thermal CVD (FIG. 12C). Here, at the timethe silicon layer 1304 is formed, a dopant may be introduced in a tracequantity to control the silicon layer to be of a p⁻-type (or n⁻-type).On the silicon layer 1304, an n⁺-type layer (or p⁺-type layer) 1305 isdeposited by plasma CVD or by increasing the dopant when the formationof the silicon layer 1304 (FIG. 12D) is finished.

[0124] The single-crystal silicon substrate 1301 having the siliconlayers 1304 and 1305 is set in an anodizing apparatus shown in FIG. 15,at its prescribed position such that the silicon layer 1305 facesnegative electrodes 1604 and 1606 in the aqueous HF solution. Here, thenegative electrodes 1604 and 1606 have substantially the same form aspositive electrodes 1605 and 1607, respectively, coming in contact withthe back of the single-crystal silicon substrate 1301. The electrodes1604 and 1605 are provided along the periphery of the single-crystalsilicon substrate 1301 in the form of, e.g., a beltlike ring or polygonas shown in FIG. 16. The electrodes 1606 and 1607 are positioned insidethe electrodes 1604 and 1605, respectively, in the region other than theperiphery of the single-crystal silicon substrate 1301 and in the formof, e.g., a disk or polygon as shown in FIG. 17.

[0125] A relatively high electric current is flowed across theelectrodes 1604 and 1605 and etching is carried out in anelectropolishing mode to remove the silicon layers 1304 and 1305, or thesilicon layers 1304 and 1305 and part or the whole of the porous layer1303, lying on the periphery of the single-crystal silicon substrate1301. In this case, the removing of either only the silicon layers 1304and 1305 or both the silicon layers 1304 and 1305 and part or the wholeof the porous layer 1303 may be selected in accordance with theseparating strength of the porous layer.

[0126] In the case where as shown in FIG. 3B the silicon layers 1304 and1305 (304) do not completely cover the porous layer 1303 (303) and theporous layer 1303 is uncovered to the surface of the single-crystalsilicon substrate 1301 (wafer), part or the whole of the porous layer1303 at its uncovered portion is removed. A relatively low electriccurrent is flowed across the electrodes 1604 and 1605 to carry outconventional anodizing, and a thin porous layer 1309 is formed on thesurface of the silicon layer 1304 in its region other than theperipheral region of the single-crystal silicon substrate 1301 toprovide an anti-reflection layer (FIG. 12E). Here, the step of removingthe silicon layer and porous layer on the periphery and the step offorming the porous layer on the surface of the silicon layer in itsregion other than the peripheral region may be carried outsimultaneously or separately.

[0127] After a grid electrode 1310 is formed on the surface of theporous layer 1309, a transparent polymeric-film substrate 1306 is bondedwith an adhesive 1307 to the side of the single-crystal siliconsubstrate 1301 on which the silicon layers 1304 and 1305 have beenformed, and these are put in an oven (not shown) and heated to bring thepolymeric-film substrate 1306 and the silicon layer 1305 on thesingle-crystal silicon substrate 1301 into firm bond (FIG. 12F).

[0128] Next, the transparent polymeric-film substrate 1306 and thesingle-crystal silicon substrate 1301 with the stated layers, thusfirmly bonded, are put in a water bath to make ultrasonic waves actthereon (not shown). Thus, the silicon layer 1304 is separated from thesingle-crystal silicon substrate 1301 at the part of the porous layer1303 (FIG. 12G).

[0129] A porous layer 1303 a remaining on the silicon layer 1304separated from the single-crystal silicon substrate 1301 is selectivelyremoved. Only the porous silicon is removed by electroless wet chemicaletching by the use of at least one of a usual silicon etchant,porous-silicon selective-etchant hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to hydrofluoric acid, buffered hydrofluoric acid, a mixed solutionprepared by adding at least one of an alcohol and hydrogen peroxidewater to buffered hydrofluoric acid, and an alkali solution of KOH, NaOHor hydrazine.

[0130] On the back of the silicon layer 1304 from which the porous layerhas been removed, a p⁺-type (or n⁺-type) layer 1308 is formed and a backelectrode 1311 is formed by vacuum deposition to make up a solar cell(FIG. 12H). Here, in contact with the back electrode 1311, anothersupporting substrate (metal substrate) may optionally be bonded via aconductive adhesive.

[0131] The single-crystal silicon substrate 1301 after separation may betreated in the same manner as the above to remove a porous layer 1303 bremaining on its surface. In a case where the surface is too rough forits flatness to be tolerable, the surface may optionally be flatted(FIG. 12I), and thereafter the substrate is reused in the step of FIG.12A.

[0132] As described above, according to the present invention, after thethin-film semiconductor layer has been formed on the porous layer, thethin-film semiconductor layer and optionally the porous layer lying onthe periphery of the separating region are kept removed before theformer is bonded to the supporting substrate to which it is to betransferred. This enables the separating force to act in a goodefficiency at the portion readily separable in the porous layer(inclusive of the interface between it and the substrate orsemiconductor layer), and hence enables separation free of any adverseeffect such as cracking or breaking of the thin-film semiconductorlayer. Thus, thin-film semiconductor members having good characteristicscan be obtained in a good efficiency.

[0133] In addition, according to the present invention, while after thethin-film semiconductor layer has been formed on the porous layer thethin-film semiconductor layer and optionally the porous layer lying onthe periphery of the separating region are kept removed before theformer is bonded to the supporting substrate to which it is to betransferred, the anti-reflection layer is kept formed on the surface ofthe semiconductor layer in the separating region. This enablesseparation of the thin-film semiconductor layer having theanti-reflection layer formed beforehand, without causing any cracks orbreaks on the periphery. Thus, thin-film semiconductor members havinggood characteristics can be obtained through a process with simplifiedsteps.

[0134] In the anodizing for forming the porous layer which serves as aseparating layer or peeling layer used in the present invention, theaqueous HF solution is used, where layer can be made porous when it hasan HF concentration of at least 10% by weight. The quantity of electriccurrent flowed at the time of anodizing may appropriately be determinedin accordance with the HF concentration, the intended layer thickness ofthe porous silicon layer and the state of porous layer surface. Statedroughly, it may suitably be within the range of from 1 mA/cm² to 100mA/cm². In the course of the anodizing, the level of anodizing electriccurrent may be changed. This makes it possible to previously provide theporous layer with a structural change in density. Thus, its porousstructure can be made plural into two or more layers to enable easyseparation.

[0135] With the addition of the alcohol such as ethyl alcohol to theaqueous HF solution, bubbles of reaction product gases generated at thetime of anodizing can instantaneously be removed from the reactionliquid surface without stirring and the porous silicon can be formeduniformly and in a good efficiency. The quantity of the alcohol to beadded may appropriately be determined in accordance with the HFconcentration, the intended layer thickness of the porous silicon layerand the state of porous layer surface. It must be determined especiallywhile paying attention not to make the HF concentration too low.

[0136] In the anodizing apparatus used in the present invention to etchthe semiconductor layer at its peripheral portion or to form theanti-reflection layer at the surface of the semiconductor layer in itsregion other than the periphery, the electric currents applied acrossthe electrodes for removing the peripheral portion and across theelectrodes for forming the anti-reflection layer may preferablyindependently be controlled. As the form of the electrodes, those havingforms as shown in FIGS. 16 and 17 may preferably be used which arefitted to the form the substrate to be treated has, and the electrodesfor forming the anti-reflection layer may preferably be so disposed asto be inside the electrodes for removing the peripheral portion. Also,as occasion calls, a form in which the middle area of a disk has beenhollowed out in square as shown in FIG. 18 and a form correspondingsubstantially to the hollowed-out square as shown in FIG. 19 may beemployed in the electrodes for removing the peripheral portion and theelectrodes for forming the anti-reflection layer, respectively. Usingelectrodes having such forms, a semiconductor layer having a square formcan be separated from a substrate having the form of a round wafer. Asmaterials for the electrodes, there are no prescriptions on those forthe anode side, but those endurable to acids such as hydrofluoric acidare preferred for those on the cathode side, and platinum may mostpreferably be used.

[0137] An anodizing apparatus having an isolator 1412 (having across-sectional form corresponding to the form of the electrodes forforming the anti-reflection layer) as shown in FIG. 13 may also be usedfor the purpose of improving the independency of electric currentcontrol between the electrodes for removing the peripheral portion andbetween the electrodes for forming the anti-reflection layer.

[0138] There are no particular prescriptions on the distance between thesubstrate to be treated and the electrode on the cathode side. Withregard to the electrodes for removing the peripheral portion, since arelatively high electric current is flowed thereto, it is preferred thatthe cathode-side electrode is disposed at a position close to thesubstrate as far as possible in order to make a loss less occur and thedistance between the electrodes is shorter than that between theelectrodes for forming the anti-reflection layer. With regard to theelectrodes for forming the anti-reflection layer, the electrode on thecathode side may be at any desired distance to the substrate (see FIGS.13 to 15).

[0139] As conditions for providing the electropolishing mode to etch thesemiconductor layer and optionally the porous layer at their peripheralportions, the etching can be effected at an HF concentration of at least20% by weight when silicon is etched with the aqueous HF solution. Todilute hydrogen fluoride (HF), electroconductivity-providing agentsincluding alcohols such as ethyl alcohol, water, acids or salts thereofmay be used. The quantity of electric current flowed here mayappropriately be determined in accordance with the HF concentration.Stated roughly, it may suitably be within the range of from 10 mA/cm² to100 mA/cm².

[0140] The crystal growth process used in the present invention to formthe silicon layer on the porous layer may include thermal CVD, LPCVD(low-pressure CVD), sputtering, plasma CVD, photo-assisted CVD, andliquid-phase epitaxy. As material gases used in the case of, e.g.,gas-phase epitaxy such as the thermal CVD, LPCVD, plasma CVD orphoto-assisted CVD, they may typically include silanes such as SiH₂Cl₂,SiCl₄, SiHCl₃, SiH₄, Si₂H₆, SiH₂F₂ and Si₂F₆, and halogenated silanes.

[0141] In addition to the above material gases, hydrogen (H₂) is addedas a carrier gas or for the purpose of providing a reducing atmosphereto accelerate crystal growth. The proportion of the material gases andhydrogen may appropriately be determined in accordance with the methodsof formation, the types of material gases and also the conditions forformation. It may suitably be from 1:10 to 1:1,000 (feed flow rateratio), and more preferably from 1:20 to 1:800.

[0142] In the case where the liquid-phase epitaxy is used, silicon isdissolved in a solvent such as Ga, In, Sb, Bi or Sn to effect epitaxialgrowth while cooling the solvent gradually or providing a temperaturedifference in the solvent.

[0143] As for temperature and pressure in the epitaxial growth processused in the present invention, they may differ depending on the methodsof formation and the types of materials (gases) used. With regard to thetemperature, it may suitably be from 800° C. to 1,250° C. inapproximation when silicon is grown by usual thermal CVD, and may morepreferably be controlled to from 850° C. to 1,200° C. In the case of theliquid-phase epitaxy, the temperature depends on the types of thesolvent, and may preferably be controlled to from 600° C. to 1,050° C.when silicon is grown using Sn or In as the solvent. In low-temperatureprocesses such as plasma CVD, it may suitably be from 200° C. to 600° C.in approximation, and may more preferably be controlled to from 200° C.to 500° C.

[0144] Similarly, with regard to the pressure, it may suitably be from10⁻² Torr to 760 Torr in approximation, and more preferably be withinthe range of from 10⁻¹ Torr to 760 Torr.

[0145] The substrate to which the thin-film crystal semiconductor layeris transferred in the process of the present invention may be any ofthose having a rigidity and those having a plasticity. For example, itmay include silicon wafers, SUS stainless steel sheets, glass sheets,and plastic or resin films. As resin film materials, polymeric films maypreferably be used, including as typical ones polyimide film, EVA(ethylene vinyl acetate) film, and Tefzel.

[0146] As methods for bonding the substrate to the thin-film crystalsemiconductor layer in the present invention, a method may preferably beused in which a conductive metal paste such as copper paste or silverpaste, a mixture of such a conductive metal paste with glass frit, or anepoxy type adhesive is inserted between the both to bring them intoadhesion, followed by burning to effect firm bond. In this case, themetal such as copper or silver sintered by burning functions also as aback electrode and a back reflection layer. Also, in the case of thesubstrates such as polymeric films, the substrate and the thin-filmcrystal semiconductor layer are brought into adhesion and in this state(here, a back electrode is previously formed on the surface of thethin-film crystal semiconductor layer) the temperature is raised to thesoftening point of the film substrate to bond the both firmly.

[0147] Methods for separating the semiconductor layer whose peripheralportion has been removed include a method in which a mechanical externalforce is made to act directly between the substrates to make separationat the part of the porous layer as a separating layer, and a method inwhich force (internal stress) existing in the semiconductor layer andseparating layer, or between these and the substrate, or energy such asheat, ultrasonic waves, electromagnetic waves or centrifugal forceapplied externally is utilized and made to act indirectly on a brittleportion in the separating layer.

[0148] In the solar cell according to the present invention, the surfaceof the semiconductor layer may be subjected to texture treatment inorder to make incident light less reflect. In the case of silicon, thetreatment is made using hydrazine, NaOH or KOH. The height of pyramidsof the texture formed may suitably be within the range of from severalmicrons to tens of microns.

EXAMPLES

[0149] The present invention will be described below in greater detailby giving Examples specifically.

Example 1

[0150] The present Example concerns production of a thin-film solar cellaccording to the process shown in FIGS. 1A to 1H.

[0151] Into the surface layer of one side of a p-type single-crystalsilicon substrate (wafer) 101 of 800 μm thick and 4 inches diameter, B(boron) was introduced by thermal diffusion to form a p⁺-type layer 102.This substrate was subjected to anodizing in the state its side on whichthe p⁺-type layer was not formed was shielded from an anodizing solutionand while changing electric currents into two stages, to obtain a porouslayer 103 of about 13 μm thick. The electric currents were first flowedat 8 mA/cm² for 10 minutes and thereafter flowed at 30 mA/Cm² for 1minute. Because of the changing of electric currents, the porous layerwas formed in double-layer structure consisting of a porous layer with adense structure and a porous layer with a coarse structure.

[0152] Next, the p-type single-crystal silicon wafer 101 at the surfaceof which the porous layer 103 was formed was annealed at a surfacetemperature of 1,050° C. for 1 minute in an atmosphere of hydrogen, andwas thereafter immersed in a 900° C. metallic solution of indium intowhich silicon had been dissolved to become supersaturated, followed byslow cooling to form the silicon layer 104 in a thickness of 30 μm.Here, a cover was provided on the porous layer such that the siliconlayer 104 was formed only in the region of a concentric circle smallerby 7 mm in diameter than the wafer.

[0153] Next, to the surface of the silicon layer 104, P (phosphorus) wasdiffused to form an n⁺-type layer, and thereafter nine solar-cellregions each having an area of 1 cm² were fabricated at the centralportion of the silicon layer 104 by isolation of the n⁺-type layer, andan electrode and an anti-reflection layer were further formed. Thesilicon substrate 101 with these was set in a chamber of a reactive ionetching apparatus, and a glass protective mask of 90 mm diameter wasplaced thereon in center alignment with the wafer. The portion protrudedfrom the glass mask was etched on the surface side of the silicon layerto remove the silicon layer and part of the porous layer at thatportion. These were removed in a depth of about 11 μm, which reached thesecond-layer porous layer by about a half of its layer thickness. Atransparent adhesive 105 was coated on the surface of the remainingsilicon layer 104 such that it was not forced out to the portionuncovered after removal as well as the side (lateral surface) uncoveredafter removal, and then a transparent substrate 106 was firmly bondedthereto. Thereafter, force was made to act on the porous layer toseparate the silicon layer 104 from the silicon substrate 101, and aback electrode was formed to make up a solar cell.

[0154] Compared with a solar cell produced by separating the siliconlayer 104 without removing the peripheral portion of the separatingregion of the silicon substrate 101, solar cells were obtainable in agood yield because the silicon layer was less cracked or broken, andalso high values were obtainable on their photoelectric conversionefficiency.

Example 2

[0155] The present Example concerns production of a semiconductor memberaccording to the process shown in FIGS. 2A to 2H.

[0156] A porous layer 203 of about 13 μm thick was formed in the samemanner as in Example 1 except that a p⁺-type layer 202 was formed at thesurface of a p-type silicon substrate (wafer) 201 of 5 inches diameterand the wafer was shielded from the anodizing solution on its periphery5 mm inward the semiconductor growth surface from the peripheral edge ofthe wafer. Thereafter, a silicon semiconductor layer 204 of 0.5 μm thickwas epitaxially grown by CVD. (FIGS. 2A to 2C).

[0157] Next, the surface layer of the semiconductor layer 204 wassubjected to thermal oxidation to form an SiO₂ layer, and thereafter asame-type quartz glass substrate 206 having a diameter smaller by 15 mmthan the silicon substrate 201 was bonded thereto in its centeralignment with the wafer under heat treatment at 700° C. for 0.5 hours(FIG. 2D). Then the portion protruded from the glass substrate 206(i.e., part of the semiconductor layer 204, part of the porous layer 203and the semiconductor layer 202) was removed by reactive ion etching(FIG. 2E). It was removed in a depth of about 23 μm from the surface onthe side of the substrate. The glass substrate 206 was irradiated withultrasonic waves to break the porous layer 203 to effect separation, sothat the thin-film semiconductor layer was transferred onto the glasssubstrate 206. Also, a porous residue 207 remaining on the surface ofthe semiconductor layer 204 was removed by etching to obtain an SOI(silicon-on-insulator) member 208.

[0158] In visual examination, neither cracks nor breaks were observableat the peripheral portion of the semiconductor layer. As a result offurther observation with a transmission electron microscope, it wasconfirmed that any additional defects were not seen to have been broughtinto the layer and a good crystal state was obtained.

Example 3

[0159] On both sides of a polycrystalline silicon wafer 501 (FIG. 5) of1 mm thick and 4 inches diameter, p⁺-type layers were formed andthereafter this wafer was subjected to anodizing on its both sides inthe state the wafer was shielded from an anodizing solution in the samemanner as in Example 2. Electric currents were first flowed at 8 mA/cm²for 10 minutes and thereafter flowed at 32 mA/cm² for 1 minute. Sincethe values of electric currents were set under conditions different formthose in Example 1, porous layers were formed in a lower strength. Theporous layers formed in double-layer structure on both sides of thewafer were each in a layer thickness of about 12 μm.

[0160] Subsequently, on each porous layer on the both sides of thewafer, an n⁺-type semiconductor layer of about 0.2 μm thick and ap⁺-type semiconductor layer of 30 μm were successively epitaxially grownby immersing the wafer with porous layer in liquid-phase epitaxysolutions in which impurities suited respectively to these layers hadbeen dissolved. Aluminum substrates 502 (406 in FIG. 4) serving also asback electrodes and supporting substrates, having a size smaller by 7 mmin radius than the wafer, were thermally fused to the semiconductorlayers on the both sides and simultaneously aluminum was diffused toform p⁺-type layers. Thereafter, each aluminum substrate 502 wasprotected by masking with a material resistant to hydrofluoric acid, andthe uncovered portions of the semiconductor layers were removed byetching with a hydrofluoric acid/nitric acid (HF:HNO=3:1) etchant 505.The etching was carried out using a jig as shown in FIG. 5. A holder 503is joined to wafer holders (not shown) so that the wafer can be heldbetween them. The main body of the jig also has the function to enableadjustment of height so that only the portion intended to be etched isimmersed in the etchant. After the semiconductor layers on the bothsides at their peripheral portions were removed by etching, the maskingof the aluminum substrates was removed. Then, as shown in FIG. 4, forcewas applied to the porous layers 403 while holding with the jig 409 theregions 408 in wafer 401 from which the semiconductor layers 404 wereremoved, thus the semiconductor layers 404 on the both sides wereseparated. As a result, the semiconductor layers 404 were respectivelytransferred to aluminum substrates 406. Porous residues having remainedon the surfaces of the semiconductor layers were removed, followed byisolation, where four cells each having an area of 4 cm² werefabricated, grid electrodes were formed and TiO₂ anti-reflection layershaving also the passivation effect were deposited to make up solarcells.

[0161] Compared with solar cells having the same construction butproduced by separating the semiconductor layers without removing theirperipheral portions, those produced by separating the semiconductorlayers after the removing of their peripheral portions less caused thedefects such as cracks or breaks by separation, and hence the yield ofcells was good and also high values were obtainable on their conversionefficiency.

[0162] Porous residues having remained on the wafer surfaces after theliquid-phase epitaxial thin films were separated were also removed, andthe wafer was again subjected to the like steps. As a result, solarcells having a high conversion efficiency were obtainable without anyproblem.

Example 4

[0163] One side of a single-crystal silicon wafer 601 (FIG. 6) of 1 mmthick and 5 inches diameter was subjected to anodizing under the sameconditions as in Example 3 to form a porous layer 602 of 12 μm thick indouble-layer structure. The same wafer as the above on one side of whicha porous layer was formed in the same way was additionally prepared.These two wafers were closely put together face to face on their sideopposite to the side on which the porous layer was formed, and theirperiphery was covered and fixed with a jig such that any liquid-phaseepitaxy solution did not enter their side on which the porous layer wasnot formed. These were immersed in a liquid-phase epitaxy solution togrow a p⁻-type semiconductor layer 603 epitaxially in a thickness ofabout 40 μm on each porous layer.

[0164] Next, the wafers with these layers were detached from the jig. Oneach semiconductor layer and in its region of 75 mm×75 mm, a comb-likepattern 604 with fingers of 80 μm wide and 100 μm in pitch whichextended form a bus bar of 3 mm wide was formed by screen printing usinga paste containing aluminum. Then, the aluminum-silicon contact surfaceand its vicinity was made into alloy at 900° C. to form a p⁺-type layer605 and simultaneously the whole surface was oxidized. Only the partwhere silicon surface was oxidized was selectively removed by etchingwithout removing oxide film 606 of aluminum. Thereafter, an n⁺-typesemiconductor layer 607 was deposited on the surface by CVD.

[0165] Next, the region where the pattern of p⁺-type layer 605 andn⁺-type layer 607 was formed was bonded with a conductive paste 608, toa supporting substrate 609 (701 in FIG. 7) of 77 mm×77 mm made ofstainless steel. Thereafter, the semiconductor layer at its portion 702protruded from the supporting substrate was removed by grinding andpolishing. After this removal, a tensile force was applied to theinterface between the supporting substrate made of stainless steel andthe wafer to separate the thin-film semiconductor layer. A porousresidue having remained on the thin-film semiconductor layer was removedand an anti-reflection layer was formed. Thus, back-sidejunction-concentrated type solar cells were produced.

[0166] Characteristics of the solar cells were evaluated. As a result,compared with solar cells having the same construction but produced byseparating the semiconductor layers without removing their peripheralportions, superior characteristics were obtainable.

Example 5

[0167] On one side of a single-crystal wafer of 5 inches diameter, aporous layer was formed in the same manner as in Example 1. Thereafter,on the porous layer, a p⁺-type semiconductor layer of about 1 μm thickand a p⁻-type semiconductor layer of 30 μm thick were successivelyepitaxially grown by liquid-phase growth in the same manner as inExample 4, by immersing the wafer with porous layer in liquid-phaseepitaxy solutions in which impurities suited respectively to theselayers had been dissolved. Thereafter, a diffusing agent was coated onthe surface of the p⁻-type semiconductor layer, followed by heattreatment to form an n⁺-type layer.

[0168] Subsequently, the n⁺-type layer, formed on the whole surface, wassubjected to isolation in a region 801 (FIG. 8) of 85 mm×85 mm. Then anelectrode pattern was printed with a silver paste on the surface of then⁺-type layer, and an insulating anti-reflection layer was furtherdeposited thereon.

[0169] Next, using a YAG laser, the semiconductor layer and porous layerwere removed in a depth of 45 μm in parallel to a pair of opposing sidesof the square region 801 formed by isolation as shown in FIG. 8, and inregions 802 set apart by 5 mm from isolation lines. A transparentadhesive was so coated as not to come around to the portion uncoveredafter removal with the laser, and a transparent substrate was bonded tothe semiconductor layer. Then, force was applied to the substrate tocause separation to progress in the direction parallel to the sideregions removed by the laser. Thus, the semiconductor layer wastransferred to the transparent substrate, followed by formation of aback electrode to obtain a solar cell.

[0170] In the present Example, too, compared with those having the sameconstruction but produced by separating the semiconductor layers withoutremoving their peripheral portions, solar cells having less breaks ofthin films and having superior characteristics were obtainable.

Example 6

[0171] One side of a single-crystal silicon wafer 901 (FIG. 9) of 1 mmthick and 5 inches diameter was subjected to anodizing under the sameconditions as in Example 1 to form a porous layer 902 of 12 μm thick indouble-layer structure. The same wafer as the above on one side of whicha porous layer was formed in the same way was additionally prepared.These two wafers were closely put together face to face on their sideopposite to the side on which the porous layer was formed, and theirperiphery was covered and fixed with a jig such that any liquid-phaseepitaxy solution did not enter their contact surface. These wereimmersed in a liquid-phase epitaxy solution to grow a p⁻-typesemiconductor layer (silicon layer) 903 epitaxially in a thickness ofabout 40 μm on each porous layer.

[0172] Next, the wafers with these layers were detached from the jig. Tothe surface of each silicon layer, P (phosphorus) was diffused to forman n⁺-type layer, and thereafter nine solar-cell regions each having anarea of 1 cm² were fabricated at the central portion of the siliconlayer by isolation of the n⁺-type layer, and an electrode and ananti-reflection layer were further formed.

[0173] Subsequently, a glass substrate 905 of the same type as thesilicon wafer was bonded with a transparent adhesive 904. Here, thequantity of the adhesive was so controlled that the adhesive did notcome around to the edge face of the wafer. Against the edge face of thesilicon wafer thus bonded to the glass substrate, a fluid 906 of mixtureof fine glass beads with pure water was jetted at a jet-out pressure of1.0 to 2.0 kg/cm² by means of a honing apparatus. Thus, the porous layerformed at the edge of the wafer was removed over the whole periphery ofthe wafer. The glass substrate was provided with a mask tape 907 so asnot to be scratched. Thereafter, a wedge made of stainless steel andcoated with Teflon was inserted to the porous layer to separate thethin-film semiconductor layer from the silicon wafer at the part of theporous layer. A porous residue having remained on the thin-filmsemiconductor layer was removed and a back electrode was formed. Thus,solar cells were produced.

[0174] Characteristics of the solar cells were evaluated. As a result,compared with solar cells having the same construction but produced byseparating the semiconductor layers without removing their peripheralportions, superior characteristics were obtainable.

Example 7

[0175] The present Example concerns production of a semiconductor memberby transferring a single-crystal silicon layer to a glass substrateaccording to the process shown in FIGS. 10A to 10H.

[0176] Into the surface layer of a silicon wafer 1101 of 5 inchesdiameter, boron was thermally diffused using BCl₃ as a thermal diffusionsource at a temperature of 1,200° C. to form a p⁺-type layer to obtain adiffusion layer 1102 of about 3 μm thick (FIG. 10A). This wafer whosesurface layer 1102 became p⁺-type was subjected to anodizing in anaqueous HF/C₂H₅OH solution under conditions shown in Table 1 (FIG. 10B).Here, the anodizing was carried out first at a low electric current of 8mA/cm² for 2.5 minutes and thereafter, slowly raising the level ofelectric current on, the anodizing was stopped at the time the electriccurrent reached 35 mA/cm² in 30 seconds. The porous layer 1103 formedwas in a layer thickness of about 3 μm in total. TABLE 1 Anodizingsolution HF:H₂O:C₂H₅OH = 1:1:1 Current density 8 mA/cm² → 35 mA/cm²Anodizing time 2.5 min. → (30 sec.) → 0 sec.

[0177] This makes it possible to previously provide the porous layerwith an internal structural change in density, and later enables easyseparation of the silicon layer 1104 from the wafer 1101.

[0178] Next, on the surface layer 1103 having been made porous, asilicon layer 1104 was formed in a thickness of 0.5 μm by thermal CVD(FIG. 10C). Here, the peripheral portion of the wafer was in such astate that the silicon layer 1104 covered the top of the porous layer1103 like that shown in FIG. 3A.

[0179] The wafer 1101 with porous layer was set in the anodizingapparatus shown in FIG. 14, at its prescribed position such that thesilicon layer 1104 faced a negative electrode 1504 in the aqueous HFsolution. Here, the negative electrode 1504 had substantially the sameform as a positive electrode 1505 coming in contact with the back of thewafer 1101, and was provided along the periphery of the wafer 1101 inthe form of a beltlike ring (see FIG. 16). An electric current of 120mA/cm² was flowed across the electrodes in an HF/H₂O solution(HF:C₂H₅OH:H₂O=1:1:6) and etching was carried out in an electropolishingmode to remove the silicon layer 1104 and part of the porous layer 1103,lying on the periphery of the wafer 1101, in a depth of 10.5 μm in total(FIG. 10D).

[0180] On the surface of the silicon layer 1104, an oxide film 1106 wasformed in a thickness of 0.1 μm by normal-pressure CVD at 450° C.Thereafter, a glass substrate 1105 was bonded to the oxide film 1106,and these were put in a heat-treating furnace (not shown) and heated at1,150° C. to bring the glass substrate 1105 and the silicon layer 1104on the wafer 1101 into firm bond (FIG. 10E).

[0181] Next, force was made to act between the supporting substrate 1105and the wafer 1101 in the direction where they were pulled apart fromeach other, to separate them at the part of the porous layer 1103 (FIG.10F). Neither cracks nor breaks were seen on the periphery of thesilicon layer thus separated.

[0182] A porous layer 1103 a remaining on the silicon layer 1104separated from the wafer 1101 was selectively removed with a solution ofHF/H₂O mixture. Thus, an SOI member was obtained (FIG. 10G).

[0183] The wafer 1101 after separation was treated in the same manner asthe above to remove a porous layer 1103 b remaining on its surface. In acase where the surface was too rough for its flatness to be tolerable,the surface was optionally flatted by polishing or the like (FIG. 10H).

[0184] Using the regenerated wafer thus obtained, the above steps wererepeated to obtain a plurality of semiconductor (SOI) members havinghigh-quality semiconductor layers.

Example 8

[0185] The present Example concerns production of a solar cell bytransferring a thin-film single-crystal silicon layer to a polyimidefilm according to the process shown in FIGS. 11A to 11I.

[0186] Into the surface layer of a silicon wafer 1201 of 5 inchesdiameter, boron was thermally diffused using BCl₃ as a thermal diffusionsource at a temperature of 1,200° C. to form a p⁺-type layer to obtain adiffusion layer 1202 of about 3 μm thick (FIG. 11A). This wafer whosesurface layer 1202 became p⁺-type was subjected to anodizing in anaqueous HF/C₂H₅OH solution under conditions shown in Table 2. Here, theanodizing was carried out first at a low electric current of 8 mA/cm²for 10 minutes and thereafter, raising the level of electric current, anelectric current was flowed at 30 mA/cm² for 1 minute. The porous layer1203 formed was in a layer thickness of about 13 μm in total. TABLE 2Anodizing solution HF:H₂O:C₂H₅OH = 1:1:1 Current density 8 mA/cm² → 30mA/cm² Anodizing time 10 min. → (0 sec.) → 1 min.

[0187] Next, on the surface of the porous layer 1203, a silicon layer1204 was formed in a thickness of 30 μm by epitaxial growth carried outunder conditions shown in Table 3, by means of a liquid-phase epitaxysystem of a slider type making use of indium as a solvent. Here, boronwas added in the solvent in a trace quantity (approximatelyzero-point-few ppm to few ppm based on the weight of silicon dissolvedtherein) to make the grown silicon layer 1204 into p⁻-type and also,after the growth was completed, a p⁺-type layer 1205 was further formedon grown silicon layer 1204 in a thickness of 1 μm, using another indiummelt having a larger boron content (at least hundreds of ppm based onthe weight of silicon dissolved therein) (FIGS. 11C, 11D). Here, inrelation to a jig on which the substrate was placed, the peripheralportion of the wafer was not in contact with the indium solvent, andhence the silicon layer 1204 did not completely cover the porous layer1203 like that shown in FIG. 3B and the porous layer 1203 stooduncovered at the surface of the substrate 1201. TABLE 3 H₂ flow rate 5liter/min. Solvent (In) temp. 950° C. → 830° C. Slow-cooling rate 1°C./min.

[0188] The wafer 1201 with porous layer was set in the anodizingapparatus shown in FIG. 14, at its prescribed position such that thesilicon layer 1204 faced a negative electrode 1504 in the aqueous HFsolution. Here, the negative electrode 1504 had substantially the sameform as a positive electrode 1505 coming in contact with the back of thewafer 1201, and was provided along the periphery of the wafer 1201 inthe form of a beltlike ring (see FIG. 16).

[0189] An electric current of 170 mA/cm² was flowed across theelectrodes in an HF/H₂O solution (HF:C₂H₅OH:H₂O=1:1:6) and etching wascarried out in an electropolishing mode to remove the silicon layer 1204and the whole (layer thickness: 13 μm) of the porous layer 1203, lyingon the periphery of the wafer 1201 (FIG. 11E).

[0190] On one side of a polyimide film 1206 of 50 μm thick, a silverpaste 1207 was coated in a thickness of 10 to 30 μm by screen printing,and this side was brought into close contact with the p⁺-type siliconlayer 1205 side to effect bonding. In this state, these were put in anoven, where the silver paste was sintered under conditions of 360° C.for 20 minutes and also the polyimide film 1206 and the silicon layer1205 on the wafer 1201 into firm bond (FIG. 11F).

[0191] Against the polyimide film 1206 and wafer 1201 brought into abonded structure, being kept fastened with a vacuum chuck (not shown) onthe latter's side not bonded to the former, force was made to act fromone edge of the polyimide film 1206. The flexibility of the polyimidefilm was utilized and the both were slowly drawn off from an edge of thewafer 1201 to effect-separating. Thus, the silicon layers 1204 and 1205were separated from the wafer 1201 at the part of the porous layer 1203and transferred onto the polyimide film 1206 (FIG. 11G). Neither cracksnor breaks were seen on the periphery of the silicon layers thusseparated.

[0192] A porous layer 1203 a remaining on the silicon layer 1204separated from the wafer 1201 was selectively etched with a solution ofHF/H₂O₂/H₂O mixture with stirring. The silicon layers remained withoutbeing etched and only the porous layer was completely removed.

[0193] The surface of the silicon layer 1204 on the polyimide film, thusobtained, was lightly etched with a hydrofluoric acid/nitric acid typeetchant to clean it, and thereafter on the silicon layer an n-type μc-Si(microcrystalline silicon) layer was deposited in a thickness of 200angstroms by means of a conventional plasma CVD system under conditionsshown in Table 4. Here, the μc-Si layer had a dark conductivity of ˜5S/cm. TABLE 4 Gas flow rate ratio SiH₄/H₂ = 1 cc/20 cc PH₃/SiH₄ = 2.0 ×10⁻³ Substrate temperature 250°C. Pressure 0.5 Torr Discharge power 20 W

[0194] Finally, on the μc-Si layer an ITO (indium tin oxide) transparentconductive film 1209 (82 nm) and a collector electrode 1210 (Ti/Pd/Ag:400 nm/200 nm/1 μm) were formed by EB (electron beam) vacuum depositionto make up a solar cell (FIG. 11H).

[0195] In regard to the thin-film single-crystal silicon solar cell onpolymide thus obtained, its I-V characteristics under irradiation bylight of AM1.5 (100 mW/cm²) were measured. As a result, open-circuitvoltage was 0.59 V, short-circuit photoelectric current was 33 mA/cm²and fill factor was 0.78 at a cell area of 6 cm², and an energyconversion efficiency of 15.2% was obtained.

[0196] The porous layer remaining on the silicon wafer after separatingwas also removed by etching in the same manner as the above, and itssurface was made flat (FIG. 11I). Using the regenerated wafer thusobtained, the above steps were repeated to obtain a plurality ofthin-film single-crystal solar cells having high-quality semiconductorlayers.

Example 9

[0197] The present Example concerns production of a solar cell bytransferring a polycrystalline silicon layer to a Tefzel film(transparent film) according to the process shown in FIGS. 12A to 12I.

[0198] Into the surface layer of a rectangular (square) polycrystallinesilicon wafer 1301 of 500 μm thick, boron was thermally diffused usingBCl₃ as a thermal diffusion source at a temperature of 1,200° C. to forma p⁺-type layer 1302 to obtain a diffusion layer of about 3 μm thick(FIG. 12A). Next, anodizing was carried out in an aqueous HF solutionunder conditions shown in Table 5, to form a porous silicon layer 1303at the surface of the wafer (FIG. 12B). More specifically, the anodizingwas carried out first at a low electric current of 5 mA/cm² for 2.5minutes and thereafter, slowly raising the level of electric current on,the anodizing was stopped at the time the electric current reached 32mA/cm² in 30 seconds. TABLE 5 Anodizing solution HF:H₂O:C₂H₅OH = 1:1:1Current density 5 mA/cm² → 32 mA/cm² Anodizing time 2.5 min. → (30 sec.)→ 0 sec.

[0199] On the surface of the porous silicon layer, a silicon layer(polycrystalline) was formed in a layer thickness of about 30 μm bycrystal growth carried out under conditions shown in Table 6, by meansof a conventional thermal CVD system. TABLE 6 Gas flow rate ratioSiH₄Cl₂/H₂ = 0.5/100 (1/min.) Substrate temperature 1,050° C. Pressurenormal pressure Growth time 30 min.

[0200] Here, during the growth, zero-point-few ppm to few ppm of B₂H₆was added to make the grown silicon layer into p⁻-type and also, at thefinal stage of the growth, hundreds of ppm of PH₃ was added in place ofB₂H₆ to form an n⁺-layer 1305 in a thickness of 0.2 μm to form a p-njunction (FIG. 12C, 12D).

[0201] Here, the peripheral portion of the wafer was in such a statethat the silicon layer 1304 covered the top of the porous layer 1303like that shown in FIG. 3A.

[0202] The wafer 1301 with porous layer was set in the anodizingapparatus shown in FIG. 15, at its prescribed position such that thesilicon layer 1305 faced a negative electrodes 1604 and 1606 in theaqueous HF solution. Here, the negative electrodes 1604 and 1606 hadsubstantially the same form as positive electrodes 1605 and 1607,respectively, coming in contact with the back of the wafer 1301. Theelectrodes 1604 and 1605 were provided along the periphery of the wafer1301 in the form of a beltlike rectangle (square) (see FIG. 16). Theelectrodes 1606 and 1607 were provided inside the electrodes 1604 and1605, respectively, in the region other than the periphery of the wafer1301 in the form of a rectangle (square) (see FIG. 17).

[0203] An electric current of 150 mA/cm² was flowed across theelectrodes 1604 and 1605 in an HF/H₂O solution (HF:C₂H₅OH:H₂O=1:1:6) andetching was carried out in an electropolishing mode to remove thesilicon layers 1304 and 1305 and the whole of the porous layer 1303,lying on the periphery of the wafer 1301. Also, an electric current of 8mA/cm² was flowed across the electrodes 1604 and 1605 to carry outconventional anodizing to form a thin porous layer 1309 at the surfaceof the silicon layer 1304 in the region other than the periphery of thewafer 1301, in a thickness of 90 nm and as an anti-reflection layer(FIG. 12E).

[0204] After the anodizing was completed, on the anti-reflection layer1309 an ITO transparent conductive film (not shown) (82 nm) and acollector electrode 1310 (Ti/Pd/Ag: 400 nm/200 nm/1 μm) were formed byEB vacuum deposition to make up a solar cell previously. Thereafter, onone side of a Tefzel film 1306 of 80 μm thick, a transparent adhesive1307 was coated in a thickness of 10 to 30 μm, and this side was broughtinto close contact with the transparent conductive film/collectorelectrode surface to effect bonding (FIG. 12F).

[0205] Upon sufficient hardening of the adhesive, the Tefzel film 1306and wafer 1301 brought into a bonded structure were fastened with avacuum chuck (not shown) on the latter's side not bonded to the former,in the state of which force was made to act from one edge of the Tefzelfilm 1306. The flexibility of the Tefzel film was utilized and the bothwere slowly drawn off from an edge of the wafer 1301 to effectseparating. Thus, the silicon layers 1304 and 1305 were separated fromthe wafer 1301 at the part of the porous layer 1303 and transferred ontothe Tefzel film 1306 (FIG. 12G). Neither cracks nor breaks were seen onthe periphery of the silicon layers thus separated.

[0206] A porous layer 1303 a remaining on the silicon layer 1304separated from the polycrystalline silicon wafer 1301 was selectivelyetched with an aqueous KOH solution of 1% by weight in concentrationwith stirring. The silicon layer 1304 remained without being etched somuch and the porous layer was completely removed.

[0207] On the back of the silicon layer 1304 on the polyimide film, thusobtained, a p-type μc-Si layer was deposited in a thickness of 500angstroms by plasma CVD under conditions shown in Table 7. Here, theμc-Si layer had a dark conductivity of ˜1 S/cm. TABLE 7 Gas flow rateratio SiH₄/H₂ = 1 cc/20 cc B₂H₆/SiH₄ = 2.0 × 10⁻³ Substrate temperature250° C. Pressure 0.5 Torr Discharge power 20 W

[0208] As a back electrode 1311, aluminum was also deposited in athickness of 0.1 μm by sputtering, and a SUS stainless steel substrate(not shown) was further bonded as a supporting substrate via aconductive adhesive to obtain a solar cell (FIG. 12H).

[0209] In regard to the thin-film polycrystalline silicon solar cell onTefzel thus obtained, its I-V characteristics under irradiation by lightof AM1.5 (100 mW/cm²) were measured. As a result, open-circuit voltagewas 0.59 V, short-circuit photoelectric current was 33.5 mA/cm² and fillfactor was 0.78 at a cell area of 6 cm², and an energy conversionefficiency of 15.4% was obtained.

[0210] The porous layer remaining on the silicon wafer after separatingwas also removed by etching in the same manner as the above, and itssurface was made flat (FIG. 12I). Using the regenerated wafer thusobtained, the above steps were repeated to obtain a plurality ofthin-film polycrystalline solar cells having high-quality semiconductorlayers.

Example 10

[0211] The present Example concerns production of a solar cell accordingto the process shown in FIGS. 11A to 11I where a form in which themiddle area of a disk has been hollowed out in square as shown in FIG.18 and a form corresponding substantially to the hollowed-out square asshown in FIG. 19 are employed in the electrodes for removing theperipheral portion and the electrodes for forming the anti-reflectionlayer, respectively.

[0212] Into the surface layer of a silicon wafer 1201 of 8 inchesdiameter, boron was thermally diffused in the same manner as in Examples7 to 9, using BCl₃ as a thermal diffusion source at a temperature of1,200° C. to form a p⁺-type layer to obtain a diffusion layer 1202 ofabout 3 μm thick (FIG. 11A). This wafer whose surface layer 1202 becamep⁺-type was subjected to anodizing in an aqueous HF/C₂H₅OH solutionunder conditions shown in Table 2 to form a porous layer 1203 at thesurface of the wafer (FIG. 11B). Here, the anodizing was carried outfirst at a low electric current of 8 mA/cm² for 10 minutes andthereafter, raising the level of electric current, an electric currentwas flowed at 30 mA/cm² for 1 minute. The porous layer 1203 formed wasin a layer thickness of about 13 μm in total.

[0213] Next, on the surface of the porous silicon layer 1203, a siliconlayer 1204 (single crystal) was formed in a layer thickness of about 30μm by epitaxial growth carried out under conditions shown in Table 6, bymeans of a conventional thermal CVD system. Here, during the growth,zero-point-few ppm to few ppm of B₂H₆ was added to make the grownsilicon layer into p⁺-type and also, at the final stage of the growth,the B₂H₆ was increased to hundreds of ppm or more to form a p⁺-layer1205 in a thickness of 1 μm (FIGS. 11C, 11D). Here, the peripheralportion of the wafer was in such a state that the silicon layer 1204covered the top of the porous layer 1203 like that shown in FIG. 3A.

[0214] The wafer 1201 with porous layer was set in the anodizingapparatus shown in FIG. 15, at its prescribed position such that thesilicon layer 1205 faced negative electrodes 1604 and 1606 in theaqueous HF solution. Here, the negative electrodes 1604 and 1606 hadsubstantially the same form as positive electrodes 1605 and 1607,respectively, coming in contact with the back of the wafer 1201. Theelectrodes 1604 and 1605 were provided along the periphery of the wafer1201 in the form of a disk the middle area of which had been hollowedout in square (see FIG. 18). The electrodes 1606 and 1607 were providedinside the electrodes 1604 and 1605, respectively, in the region otherthan the periphery of the wafer 1201 in the form correspondingsubstantially to the hollowed-out square (see FIG. 19).

[0215] An electric current of 150 mA/cm² was flowed across theelectrodes 1604 and 1605 in an HF/H₂O solution (HF:C₂H₅OH:H₂O=1:1:6) andetching was carried out in an electropolishing mode to remove thesilicon layers 1204 and 1205 and the whole of the porous layer 1203,lying on the periphery of the wafer 1201. Also, an electric current of 6mA/cm² was flowed across the electrodes 1604 and 1605 to carry outconventional anodizing to form a thin porous layer 1209 at the surfaceof the silicon layer 1204 in the region other than the periphery of thewafer 1201, in a thickness of 95 nm and as an anti-reflection layer(FIG. 11E).

[0216] On one side of a polyimide film 1206 of 50 μm thick, a silverpaste 1207 was coated in a thickness of 10 to 30 μm by screen printing,and this side was brought into close contact with the p⁺-type siliconlayer 1205 side to effect bonding. In this state, these were put in anoven, where the silver paste was burnt under conditions of 360° C. for20 minutes and also the polyimide film 1206 and the silicon layer 1205on the wafer 1201 into firm bond (FIG. 11F).

[0217] To the polyimide film 1206 and wafer 1201 brought into a bondedstructure, an energy of ultrasonic waves was applied in a water bath.For example, upon irradiation by ultrasonic waves of 25 kHz and 650 W,the silicon layers were separated from the wafer at the part of theporous layer. Thus, the silicon layers 1204 and 1205 of about 125 mmsquare were separated from the round wafer of 8 inches diameter andtransferred onto the polyimide film 1206 (FIG. 11G). Neither cracks norbreaks were seen on the periphery of the silicon layers thus separated.

[0218] A porous layer 1203 a remaining on the silicon layer 1204separated from the wafer 1201 was selectively etched with a solution ofHF/H₂O₂/H₂O mixture with stirring. The silicon layers remained withoutbeing etched and only the porous layer was completely removed.

[0219] The surface of the silicon layer 1204 on the polyimide film, thusobtained, was lightly etched with a hydrofluoric acid/nitric acid typeetchant to clean it, and thereafter on the silicon layer an n-type μc-Silayer was deposited in a thickness of 200 angstroms by means of aconventional plasma CVD system under conditions shown in Table 4. Here,the μc-Si layer had a dark conductivity of ˜5 S/cm.

[0220] Finally, on the μc-Si layer an ITO transparent conductive film1209 (82 nm) and a collector electrode 1210 (Ti/Pd/Ag: 400 nm/200 nm/1μm) were formed by EB vacuum deposition to make up a solar cell (FIG.11H).

[0221] In regard to the thin-film single-crystal silicon solar cell onpolymide thus obtained, its I-V characteristics under irradiation bylight of AM1.5 (100 mW/cm²) were measured. As a result, open-circuitvoltage was 0.60 V, short-circuit photoelectric current was 33 mA/cm²and fill factor was 0.79 at a cell area of 6 cm², and an energyconversion efficiency of 15.6% was obtained.

[0222] The porous layer remaining on the silicon wafer after separatingwas also removed by etching in the same manner as the above, and itssurface was made flat (FIG. 11I). Using the regenerated wafer thusobtained, the above steps were repeated to obtain a plurality ofthin-film single-crystal solar cells having high-quality semiconductorlayers.

[0223] The present invention has specifically been described above bygiving Examples. The present invention is by no means construedlimitative by the foregoing Examples, and is modifiable in variety. Forexample, the foregoing description concerns production of solar cells byseparating rectangular semiconductor layers from round-wafer typesubstrates, but the form of the electrodes for removing the peripheralportion and electrodes for forming the anti-reflection layer can be setas desired, and hence semiconductor layers having any desired forms canbe separated from substrates having any desired forms.

[0224] In all the foregoing Examples, the porous layer is utilized as aseparating layer, but a semiconductor member having a separating layerformed by providing a brittle portion in the interior of the wafer canalso be treated in quite the same manner as the above, by, e.g.,implanting H ions or He ions in the silicon wafer to make heattreatment. Stated specifically, for example, H ions are implanted intothe surface portion of the crystal silicon substrate under conditions of20 keV and 5×10¹⁶ cm⁻² to form the brittle layer in a depth of 0.1 μmfrom the silicon surface, and the silicon layer is formed thereon bythermal CVD in the same manner as in, e.g., Example 7. Thereafter, theremoval of peripheral portion, the separating and so forth may befollowed up likewise according to the steps of, e.g., FIGS. 10D to 10H.

[0225] As having been described above, the present invention has made itpossible to obtain in a good efficiency semiconductor members andthin-film crystal solar cells having less cracks or breaks and goodcharacteristics. This has made it possible to provide in the marketsemiconductor members and solar cells having mass productivity and goodquality. The present invention also has made it possible to form throughsimple steps thin-film crystal solar cells having good characteristics,making it possible to produce inexpensive solar cells. The presentinvention still also has made it possible to form with easesemiconductor members and thin-film crystal solar cells having anydesired forms.

What is claimed is:
 1. A process for producing a semiconductor membermaking use of a thin-film crystal semiconductor layer, the processcomprising the steps of: (1) anodizing the surface of a first substrateto form a porous layer at least on one side of the substrate; (2)forming a semiconductor layer at least on the surface of the porouslayer; (3) removing the semiconductor layer at its peripheral region;(4) bonding a second substrate to the surface of the semiconductorlayer; (5) separating the semiconductor layer from the first substrateat the part of the porous layer by applying an external force to atleast one of the first substrate, the porous layer and the secondsubstrate; and (6) treating the surface of the first substrate afterseparation and repeating the above steps (1) to (5).
 2. The process forproducing a semiconductor member according to claim 1, wherein, in thestep (3), the semiconductor layer at its peripheral region is removedtogether with the porous layer lying directly beneath that region.
 3. Aprocess for producing a semiconductor member making use of a thin-filmcrystal semiconductor layer, the process comprising the steps of: (1)anodizing the surface of a first substrate to form a porous layer atleast on one side of the substrate; (2) forming a semiconductor layer atleast on the surface of the porous layer; (3) bonding a second substrateto the semiconductor layer; (4) removing the semiconductor layer at itsregion not covered with the second substrate; (5) separating thesemiconductor layer from the first substrate at the part of the porouslayer by applying an external force to at least one of the firstsubstrate, the porous layer and the second substrate; and (6) treatingthe surface of the first substrate after separation and repeating theabove steps (1) to (5).
 4. The process for producing a semiconductormember according to claim 3, wherein, in the step (4), the semiconductorlayer at its region not covered with the second substrate is removedtogether with the porous layer lying directly beneath that region. 5.The process for producing a semiconductor member according to claim 1 or3, wherein the first substrate comprises silicon.
 6. The process forproducing a semiconductor member according to claim 1 or 3, wherein thefirst substrate comprises a single crystal.
 7. The process for producinga semiconductor member according to claim 1 or 3, wherein, in the step(2), a semiconductor junction is formed in the semiconductor layer.
 8. Aprocess for producing a solar cell making use of a thin-film crystalsemiconductor layer, the process comprising the steps of: (1) anodizingthe surface of a first substrate to form a porous layer at least on oneside of the substrate; (2) forming a semiconductor layer at least on thesurface of the porous layer; (3) removing the semiconductor layer at itsperipheral region; (4) bonding a second substrate to the surface of thesemiconductor layer; (5) separating the semiconductor layer from thefirst substrate at the part of the porous layer by applying an externalforce to at least one of the first substrate, the porous layer and thesecond substrate; and (6) treating the surface of the first substrateafter separation and repeating the above steps (1) to (5).
 9. Theprocess for producing a solar cell according to claim 8, wherein, in thestep (3), the semiconductor layer at its peripheral region is removedtogether with the porous layer lying directly beneath that region.
 10. Aprocess for producing a solar cell making use of a thin-film crystalsemiconductor layer, the process comprising the steps of: (1) anodizingthe surface of a first substrate to form a porous layer at least on oneside of the substrate; (2) forming a semiconductor layer at least on thesurface of the porous layer; (3) bonding a second substrate to thesemiconductor layer; (4) removing the semiconductor layer at its regionnot covered with the second substrate; (5) separating the semiconductorlayer from the first substrate at the part of the porous layer byapplying an external force to at least one of the first substrate, theporous layer and the second substrate; and (6) treating the surface ofthe first substrate after separation and repeating the above steps (1)to (5).
 11. The process for producing a solar cell according to claim10, wherein, in the step (4), the semiconductor layer at its region notcovered with the second substrate is removed together with the porouslayer lying directly beneath that region.
 12. The process for producinga solar cell according to claim 8 or 10, wherein the first substratecomprises silicon.
 13. The process for producing a solar cell accordingto claim 8 or 10, wherein the first substrate comprises a singlecrystal.
 14. The process for producing a solar cell according to claim 8or 10, wherein, in the step (2), a semiconductor junction is formed inthe semiconductor layer.
 15. A process for producing a semiconductormember obtained by separating a thin-film crystal semiconductor layerformed on a first substrate to transfer the former to a secondsubstrate, wherein the thin-film crystal semiconductor layer is removedby etching by electropolishing at its part on the periphery of the firstsubstrate.
 16. The process for producing a semiconductor memberaccording to claim 15, wherein a separating layer lies between the firstsubstrate and the thin-film crystal semiconductor layer, and only thethin-film crystal semiconductor layer, only the separating layer or boththe thin-film crystal semiconductor layer and the separating layeris/are removed at its/their part on the periphery of the firstsubstrate.
 17. The process for producing a semiconductor memberaccording to claim 16, wherein the separating layer comprises a porouslayer.
 18. The process for producing a semiconductor member according toclaim 16, wherein the separating layer comprises two or more porouslayers.
 19. The process for producing a semiconductor member accordingto claim 16, wherein the separating layer is formed by ion implantation.20. A process for producing a semiconductor member making use of athin-film crystal semiconductor layer, the process comprising the stepsof: (1) anodizing the surface of a first substrate at least on itsprincipal-surface side to form a porous layer; (2) forming asemiconductor layer on the surface of the porous layer; (3) removing thesemiconductor layer at its part on the periphery of the first substrateby electropolishing; (4) bonding a second substrate to the surface ofthe semiconductor layer; (5) separating the semiconductor layer from thefirst substrate at the part of the porous layer to transfer thesemiconductor layer to the second substrate; and (6) treating thesurface of the first substrate after separation and repeating the abovesteps (1) to (5).
 21. The process for producing a semiconductor memberaccording to claim 20, wherein, in the step (3), the semiconductor layerat its peripheral portion is removed together with the porous layerlying directly beneath that portion.
 22. The process for producing asemiconductor member according to claim 20, wherein the first substratecomprises silicon.
 23. The process for producing a semiconductor memberaccording to claim 20, wherein the first substrate comprises a singlecrystal.
 24. The process for producing a semiconductor member accordingto claim 20, wherein, in the step (2), a semiconductor junction isformed in the semiconductor layer.
 25. The process for producing asemiconductor member according to claim 20, which further comprises,between the steps (5) and (6), the step of forming a semiconductorjunction on the surface of the semiconductor layer having beentransferred to the second substrate.
 26. The process for producing asemiconductor member according to claim 20, wherein the second substratecomprises a flexible film, and force that acts in the direction wherethe film is separated from the first substrate is applied to the film toseparate the semiconductor layer at the part of the porous layer. 27.The process for producing a semiconductor member according to claim 26,wherein the second film comprises a resinous film.
 28. A process forproducing a solar cell obtained by separating a thin-film crystalsemiconductor layer formed on a first substrate to transfer the formerto a second substrate, wherein the thin-film crystal semiconductor layeris removed by etching by electropolishing at its part on the peripheryof the first substrate.
 29. The process for producing a solar cellaccording to claim 28, wherein a separating layer lies between the firstsubstrate and the thin-film crystal semiconductor layer.
 30. The processfor producing a solar cell according to claim 29, wherein the separatinglayer comprises a porous layer.
 31. The process for producing a solarcell according to claim 29, wherein the separating layer comprises twoor more porous layers.
 32. The process for producing a solar cellaccording to claim 29, wherein the separating layer is formed by ionimplantation.
 33. A process for producing a solar cell making use of athin-film crystal semiconductor layer, the process comprising the stepsof: (1) anodizing the surface of a first substrate at least on itsprincipal-surface side to form a porous layer; (2) forming asemiconductor layer on the surface of the porous layer; (3) removing thesemiconductor layer and the porous layer at their part on the peripheryof the first substrate by electropolishing; (4) forming a surfaceanti-reflection layer on the surface of the semiconductor layer at itspart other than that on the periphery of the first substrate; (5)bonding a second substrate to the surface of the semiconductor layer;(6) separating the semiconductor layer from the first substrate at thepart of the porous layer to transfer the semiconductor layer to thesecond substrate; and (7) treating the surface of the first substrateafter separation and repeating the above steps (1) to (6).
 34. Theprocess for producing a solar cell according to claim 33, wherein, inthe step (3), the semiconductor layer at its peripheral portion isremoved together with the porous layer lying directly beneath thatportion.
 35. The process for producing a solar cell according to claim33, wherein the first substrate comprises silicon.
 36. The process forproducing a solar cell according to claim 33, wherein the firstsubstrate comprises a single crystal.
 37. The process for producing asolar cell according to claim 33, wherein the step of removing thesemiconductor layer and porous layer at their part on the periphery ofthe first substrate and the step of forming a surface anti-reflectionlayer on the surface of the semiconductor layer at its part other thanthat on the periphery of the first substrate are carried outsimultaneously.
 38. The process for producing a solar cell according toclaim 33, wherein the step of removing the semiconductor layer andporous layer at their part on the periphery of the first substrate andthe step of forming a surface anti-reflection layer on the surface ofthe semiconductor layer at its part other than that on the periphery ofthe first substrate are carried in the same anodizing bath.
 39. Theprocess for producing a solar cell according to claim 33, wherein, inthe step (2), a semiconductor junction is formed in the semiconductorlayer.
 40. The process for producing a solar cell according to claim 33,which further comprises, between the steps (6) and (7), the step offorming a semiconductor junction on the surface of the semiconductorlayer having been transferred to the second substrate.
 41. The processfor producing a solar cell according to claim 33, wherein the secondsubstrate comprises a flexible film, and force that acts in thedirection where the film is separated from the first substrate isapplied to the film to separate the semiconductor layer at the part ofthe porous layer.
 42. The process for producing a solar cell accordingto claim 41, wherein the second film comprises a resinous film.
 43. Ananodizing apparatus comprising, at the peripheral portion of a substrateto be subjected to anodizing, a first electrode coming in contact withthe back side of the substrate and a second electrode facing the firstelectrode, interposing the substrate between them; the first electrodehaving substantially the same form as the second electrode.
 44. Theanodizing apparatus according to claim 43, wherein the first and secondelectrodes each have the form of a beltlike ring or a beltlike polygon.45. The anodizing apparatus according to claim 43, wherein the secondelectrode comprises platinum.
 46. An anodizing apparatus comprising, atthe peripheral portion of a substrate to be subjected to anodizing, afirst electrode coming in contact with the back side of the substrateand a second electrode facing the first electrode, interposing thesubstrate between them, and, in the remaining substrate region excludingthe peripheral portion, a third electrode coming in contact with theback side of the substrate and a fourth electrode facing the thirdelectrode, interposing the substrate between them; the first electrodeand third electrode having substantially the same form as the secondelectrode and fourth electrode, respectively.
 47. The anodizingapparatus according to claim 46, wherein the first and second electrodeseach have the form of a beltlike ring or a beltlike polygon.
 48. Theanodizing apparatus according to claim 46, wherein the third and fourthelectrodes each have the form of a disk or a polygon.
 49. The anodizingapparatus according to claim 46, wherein the distance between the firstand second electrodes is shorter than the distance between the third andfourth electrodes.
 50. The process for producing a semiconductor memberaccording to claim 15, wherein the thin-film crystal semiconductor layeris separated in a desired form by electropolishing etching.
 51. Theprocess for producing a solar cell according to claim 28, wherein thethin-film crystal semiconductor layer is separated in a desired form byelectropolishing etching.